Slave Flash Trigger Circuit


Using any camera in a
dull or dark environment generally requires the use of supplementary
light. This is a standard technique, and even where adequate natural
lighting exists, to take conventional film pictures with enhanced
contrast using a ‘fill-in’ flash for foreground subjects in shade. A
flash is often built into the camera body, but the internal flash is not
usually powerful enough to illuminate subjects much more that 3 m or so
from the camera.

On SLR cameras a hot-shoe is provided for
triggering an auxiliary, more powerful flash, but the small pocket
cameras are not so equipped. However, it is possible to trigger a slave
flash from the camera flash by optical means. Even so, things are not so
simple, for some cameras, e.g. Olympus, Nikon, Canon actually fire
twice, although it appears to be once to the naked eye. The first flash
sets the exposure and the second takes the picture. Help on
synchronisation requirements may be found at various websites maintained
by professional photographers.

Slave Flash Trigger Circuit

Slave Flash Trigger Circuit Diagram

See also http://www.caves.org.uk/flash/docs.html
for a series of articles with kits by a caving enthusiast. The
presented trigger circuit optically receive the camera flashes and
either fires at the same time as the first flash or has one flash delay
before triggering the slave flash. Additional counting circuitry is
required for more than one delay (covered by modified circuit not
presented here). Here’s how it works.

The response of phototransistor D5 to the external camera flash is
pulsed by a transistorised amplifier T1 into the dual flip-flop clock
IC1. One output of a flip-flop illuminates an LED
as a ‘ready’ signal. A double pole 3-position slide switch, S1, selects
none (e.g. for Kodak camera) or one (e.g. for Olympus camera) flash
delay before triggering. Both flip-flops are used in the 4013, the clock
signal derived from the flash is used (triggered on the rising clock
signal) to ‘divide by two’ and trigger the TIC206 triac on the first or
second flash.

A simple RC timed reset mechanism around R6-C4 is used with a
relatively long delay (about half a second) before resetting the entire
circuit. The advantage of the triac is that a trigger voltage of either
polarity can be handled. The 2N3906 may be replaced by its near
equivalent the BC212L. The SFH300-2 photodiode is supplied by Maplin as
part number MES NP64U. The triac may also be a TIC126D.
Author: Peter Metcalfe
Copyright: Elektor Electronics


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